A valve drive of this type that is used for the variable actuation of gas-exchange valves by means of displaceable cams and in which a single actuation element is sufficient for each cam part in order to displace the cam part in the direction of the two curved paths of the axial connecting link is known from DE 101 48 177 A1, which is considered class-forming. In that publication, two cam parts are disclosed with alternatively shaped axial connecting links, wherein the first axial connecting link has a central guide web for forming inner guide walls for the actuation element in the shape of a cylinder pin engaging in the axial connecting link and the second axial connecting link consists of merely outer guide walls.
The latter construction has the advantage that the production effort for the axial connecting link is significantly less due to the elimination of the guide web. In this configuration, however, there is considerable risk with respect to the functional reliability of the valve drive because the displacement process of the cam part is carried out completely, i.e., free from incorrect switching, only when the inertia of the cam part is sufficient to move it into its other end position, to a certain extent in free flight, after passing through the crossing region of the curved paths also without forced action of the cylinder pin. A prerequisite for sufficient inertia of the cam part is obviously a minimum rotational speed of the camshaft that is directly dependent on the friction between the cam part and the carrier shaft. A displacement of the cam part rotating below this minimum rotational speed could have the result that the cam part remains “halfway” and a cam follower acting on the gas-exchange valve is acted upon by several cams of the cam group in an uncontrolled manner and simultaneously under high mechanical loads. In addition, in this case there is no longer a possibility to later displace the cam part using the cylinder pin into one of the end positions, because then the axial allocation between the cylinder pin and the outer guide walls is no longer given.
This functional risk is indeed significantly lower in the first configuration of the axial connecting link with central guide web whose inner guide walls act as the cylinder pin for further accelerating forced guidance at lower rotational speeds of the cam part. Nevertheless, there is also the risk here that the cylinder pin does not merge into the specified curved path after passing through the crossing range, but instead collides with the end of the guide web likewise under a high mechanical load.